Alloy 625 is a solid-solution matrix-stiffened face-centered-cubic alloy at elevated, as well as room, temperatures. The strength of alloy 625 is derived from the stiffening effect of molybdenum and columbium on its nickel-chromium matrix. High tensile, creep and rupture strengh; outstanding fatigue and thermal-fatigue strength; oxidation resistance; excellent brazeability and weldability are some of the properties of this alloy. However, alloy 625 is a material with a very small hot working temperature range; and even though it is austenitic at room temperature, it is an inherently stiff material and difficult to move, especially in large section sizes.
In general, alloy 625 has good mechanical and physical properties for use as a wear surface and is resistant to salt water corrosion making it excellent for use in sea water applications. The specific properties of alloy 625 are reported in a brochure entitled "INCONEL alloy 625" by Huntington Alloys, Inc., Huntington, W. Va, a manufacturer of alloy 625. (INCONEL is a registered trademark of International Nickel Co.)
Initially, alloy 625 was developed as a sheet metal and had uses limited to sheet and tubing applications. Once it was demonstrated it could be successfully forged, it had other applications consistant with conventional forging techniques.
A proposed application of alloy 625 forgings is as a corrosion resistant protective sleeve for marine shafting. The advent of increased and expanded undersea exploration also makes it highly desirable as a high strength, corrosion resistant material that can be fabricated into structures (large section size and large mass), and/or used in rotating equipment. The operating conditions under which these large undersea structures and/or rotating equipment are subjected demand high and exacting properties.
Using conventional metal working methods, alloy 625 sleeves are formed by rolling the material into a plate, forming the plate into a sleeve, and seam welding the plate. However, the seam weld is a potential failure point for the sleeve.
For the most part, prior to the present invention, alloy 625 forgings were limited to small section size and small mass. To produce large section, large mass alloy 625 sleeves, it was necessary to weld together forged rings made by conventional methods from small or standard diameter ingots. These sleeves also suffer from the existence of welds in the finished product.
An exception to the conventional methods of forming large section, large mass sleeves discussed above is a method of forming them from large diameter ingots of alloy 625 described in the applicant's copending application filed June 22, 1986 having Ser. No. 875,479 also assigned to National Forge Company.
However, these prior methods of producing large section, large mass cylindrical sleeves from alloy 625 do not describe producing such sleeves using a full length forging technique.
The technology developed by the present invention not only provides a process for producing large section, large mass cylindrical sleeves of alloy 625 from full length workpieces, but also provides the specific thermo-mechanical procedures developed to provide uniformly high mechanical properties, high ductility, and a high fatigue limit in a product used in the corrosive sea water environment.